This invention relates to a threaded joint for an oil well type having excellent galling resistance. More particularly, it relates to a threaded joint for an oil well pipe used in the excavation of crude oil and which can undergo repeated tightening and loosening without using a liquid lubricant and without galling, which maintains air tightness, and which can be used repeatedly.
Oil well pipe such as tubing and casing which is used in the excavation of oil wells is typically connected by threaded joints. The depth of oil wells is normally 2,000 to 3,000 meters, but recently the depth has reached 8,000 to 10,000 meters in deep wells such as those in offshore oil fields. A threaded joint for connecting such oil well pipes is used under tensile forces in the axial direction caused by the weight of the oil well pipe and the threaded joint itself, a compound pressure due to internal and external surface pressure, and heat, so it is required that it be able to maintain air tightness without damage even in such an environment. Furthermore, during the process of lowering tubing or casing, there are cases in which a joint which is once tightened is loosened and then retightened. API (American Petroleum Institute) requires that there be no occurrence of seizing referred to as galling and that air tightness be maintained even if tightening (make-up) and loosening (break-out) are carried out ten times for a tubing joint and three times for a casing joint.
A threaded joint normally has a structure in which an external thread is formed on the end of an oil well pipe, an internal thread is formed on the inner surface of a threaded coupling, an unthreaded metal contact portion formed on the end of the external thread is mated with an unthreaded metal contact portion formed on the base of the internal thread, and by performing tightening, the unthreaded metal contact portions are made to contact each other and a metal seal portion is formed. Galling resistance and air tightness are increased by performing surface treatment of the threads and the unthreaded metal contact portions and by applying a compound grease at the time of tightening.
However, particularly in the unthreaded metal contact portions, a high surface pressure acts which exceeds the yield point of the material of the threaded joint, so that galling easily takes place. Thus, various threaded joints have been proposed for improving galling resistance in such portions.
For example, Japanese Published Unexamined Patent Application Sho 61-79797 discloses a threaded joint in which the threaded portions are plated with Zn, Sn, or the like, and the unthreaded metal contact portions are plated with gold, platinum, or the like.
Japanese Published Examined Patent Application Hei 3-78517 discloses a pipe joint on which is formed a film of a synthetic resin containing 20-90% of molybdenum disulfide having a particle diameter of at most 10 micrometers dispersed therein.
Japanese Published Unexamined Patent Application Hei 8-103724 discloses a surface treatment method for a steel pipe joint in which a resin film containing molybdenum disulfide is formed atop a manganese phosphate chemical formation coating.
Japanese Published Unexamined Patent Application Hei 8-105582 discloses a surface treatment method for a pipe joint in which a nitriding treatment layer is made a first layer, an iron plated layer or an iron alloy plated layer is made a second layer, and atop this is formed a third layer comprising a resin film containing molybdenum disulfide.
However, the threaded joints disclosed in the above-described publications are each premised on the use of a compound grease. This grease contains powders of heavy metals such as zinc, lead and copper, and there is the concern of the occurrence of environmental pollution when a situation develops such as the grease which is applied at the time of joining the threads being washed off or the grease overflowing to the outer surface at the time of tightening. In addition, the process of applying a compound grease worsens the working environment and also decreases the operating efficiency. Accordingly, there is a desire for the development of a threaded joint which does not use such a compound grease.
A threaded joint in which a solid lubricant film is formed on its surface has been proposed as a threaded joint which does not use a compound grease.
For example, Japanese Published Unexamined Patent Application Hei 8-233163, Japanese Published Unexamined Patent Application Hei 8-233164, and Japanese Published Unexamined Patent Application Hei 9-72467 disclose threaded joints having formed thereon a resin film in which molybdenum disulfide is dispersed in a resin.
However, a threaded joint having a resin film formed thereon has the problem that compared to when a compound grease is used, the coefficient of friction of the contacting surfaces is high at the time of tightening the threaded joint, and a large tightening force becomes necessary. In the assembly of an oil well pipe line, it is important to reduce the tightening force of the threaded joint in order to improve workability, and in order to accomplish this, a threaded joint having a low coefficient of friction at contacting surfaces and excellent lubricity is desired as a threaded joint not using a compound grease.
In recent years, there has been a demand for a heat resistant threaded joint for use in high temperature oil wells in which the environment of use has been a high temperature of 250-300xc2x0 C., and for steam injection oil wells in which high temperature steam (350xc2x0 C.) which may even reach the critical temperature is injected in order to increase the crude oil recovery efficiency. Accordingly, a threaded joint is desired to have the properties that after a heat resistance test is performed on a tightened joint at a temperature exceeding 350xc2x0 C., air tightness is maintained even when loosening and retightening is carried out.
However, with conventional technology disclosed in publications like those described above, it is difficult to guarantee such performance.
Namely, when a threaded joint is tightened, when a compound grease like that specified in API Standard BUL5A2 is employed, the grease component vaporizes due to the high temperature, and lubricity decreases, so there is the problem that a prescribed air tightness cannot be obtained at the time of retightening after loosening. Accordingly, the technology disclosed in Japanese Published Unexamined Patent Application Hei 5-117870, Japanese Published Unexamined Patent Application Hei 6-10154, Japanese Published Unexamined Patent Application Hei 5-149485, Japanese Published Unexamined Patent Application Hei 2-085593, and the like, which is premised on the application of a compound grease, has problems with respect to maintaining air tightness at high temperatures.
In the technology which is disclosed in Japanese Published Unexamined Patent Application Hei 8-233164 and Japanese Published Unexamined Patent Application Hei 9-72467, which is characterized in that it does not use compound grease, the resin deteriorates when it is exposed for long periods to a high temperature which may reach as high as 400xc2x0 C., so it fails to function as a binder holding a solid lubricant such as molybdenum disulfide, and there are problems that the lubricity decreases, improper tightening and galling occur, and air tightness worsens.
In this manner, at present, a threaded joint having excellent galling resistance which can be repeatedly used under high temperature conditions reaching as high as 400xc2x0 C. has not been obtained.
Furthermore, a threaded joint which is disclosed in the above-described publications and which does not use a compound grease has the problem that the tightening torque easily varies and tightening is unstable compared to a threaded joint using a compound grease. In addition, with threaded joints of the above-described publications which do not use a compound grease, the effect of preventing rusting of the joint in the period between shipment from a factory and on-site use is poor compared to when a compound grease is used, and there were the problems of formation of rust and accompanying swelling of a resin film, or that peeling took place and that tightening became all the more unstable at the time of tightening of the joint, galling took place, and air tightness decreased.
FIG. 1 is a schematic view showing the typical assembly of an oil well pipe and a threaded coupling at the time of shipment of the oil well pipe. Symbol A shows an oil well pipe, B shows a threaded coupling, 1 shows a box, 2 shows a pin, and 3 shows a threaded portion.
As shown in FIG. 1, an oil well pipe A is normally shipped in a state in which a box 1 having a threaded portion 3 formed on the inner surface of a threaded coupling B is threaded onto one pin 2 having a threaded portion 3 formed on the outer surface of both of its ends.
However, the exposed pin and box can easily rust. After shipment, in cases in which a compound grease is applied with the object of rust prevention, there is little formation of rust, but in cases in which only a resin film is used without a compound grease, the rust preventing properties of the film are poor, so rust easily forms.
Rust has poor lubricity, and surface irregularities can form on the pin and the box due to the formation of rust, so it can be the cause of galling at the time of tightening.
An object of this invention is to provide a threaded joint for an oil well pipe which does not use a liquid lubricant such as a compound grease and which can suppress the occurrence of galling when tightening and loosening are repeatedly performed and which can provide a high degree of air tightness and which has excellent galling resistance and air tightness.
Specifically, an object of this invention is to provide a threaded joint for an oil well pipe which can suppress the occurrence of galling when tightening and loosening are repeatedly performed without using a liquid lubricant such as a compound grease, which has a low coefficient of friction at the time of tightening, which can provide a high degree of air tightness with a low tightening torque, and which has excellent galling resistance and air tightness.
In addition, specifically, an object of the present invention is to provide a threaded joint for an oil well pipe which suppresses the occurrence of galling when repeated tightening and loosening are performed without the use of a liquid lubricant such as a compound grease and which provides a high degree of air tightness and has excellent galling resistance and air tightness for the excavation of crude oil in a high temperature environment such as a deep, high temperature oil well or a steam injection oil well.
Furthermore, a specific object of the present invention is to provide a threaded joint for an oil well pipe which suppresses the formation of rust and which can eliminate instability of tightening torque and which suppresses the occurrence of galling during repeated tightening and loosening without the use of a liquid lubricant such as a compound grease and which has excellent galling resistance and air tightness.
In order to achieve the above objects and to provide a threaded joint having a solid lubricating film formed on its surface, the present inventors performed basic investigations concerning the heat resistance and lubricity of the solid lubricant and the binder forming the solid lubricating film, and they obtained the following knowledge. Below, a solid lubricating film will be referred to simply as a lubricating film.
(1) A solid lubricant such as molybdenum disulfide or graphite does not degenerate or break down due to heat even at a temperature on the order of 400xc2x0 C., and it exhibits good lubricity which is the same as that at room temperature.
(2) If a resin is used as a binder for a solid lubricant, degeneration, decomposition, or carbonization take place in a high temperature environment after a long period exceeding 24 hours, it loses its function as a binder of covering the surface of the joint with the solid lubricant, and lubricity decreases. However, if a resin is used together with a Cu plating layer, a good lubricity is maintained even in a high temperature environment.
(3) Inorganic compounds such as oxides, carbides, or nitrides of silicon, titanium, aluminum, or the like are thermally stable, but they have utterly no ability to function as a binder for covering a solid lubricant. Accordingly, they cannot form a film, so the lubricity is low even if they are mixed in a solid lubricant.
(4) A metal alkoxide (M(OR)4, wherein M is a metal element such as silicon or titanium, 0 is oxygen, and R is an alkyl group) has an alkyl group, so it exhibits properties like an organic substance, and it can function as a binder. This material is unstable in the atmosphere, so it absorbs moisture and co-hydrolysis takes place, and the breakdown products bond to each other and transform into a thermally stable inorganic polymeric compound having a network structure with a backbone expressed by M (metal element)xe2x80x94O (oxygen). Formula (1) shows the basic molecular structure of an inorganic polymeric compound having an Mxe2x80x94O backbone. 
(5) An inorganic polymeric compound having an Mxe2x80x94O backbone is hard and has excellent wear resistance.
(6) Accordingly, by mixing a solid lubricant such as molybdenum disulfide or graphite with the above-described metal alkoxide as a binder, applying the resulting mixture to form a coating layer and performing moisturizing treatment, a lubricating film can be formed. The lubricating film comprises an inorganic polymeric compound having an Mxe2x80x94O backbone and containing a solid lubricant such as molybdenum disulfide or graphite.
The present inventors investigated suitable conditions for forming a lubricating film of an inorganic polymeric compound having the above-described Mxe2x80x94O backbone on a threaded joint and obtained the following knowledge.
(7) Galling is suppressed by forming a lubricating film of an inorganic polymeric compound having an Mxe2x80x94O backbone on the unthreaded metal contact portion of at least one of either a pin or a box.
(8) The mass ratio (B/A) of the content B of the solid lubricant to the content A of the inorganic polymeric compound having an Mxe2x80x94O backbone in the lubricating film is preferably in the range of 0.3-9.0.
(9) Galling resistance can be increased by adjusting the surface roughness Rmax in the region on which the lubricating film is formed to be 3-15 micrometers, and by making the thickness of the lubricating film 5-30 micrometers.
(10) Co-hydrolysis is promoted by performing moisturizing and heating after applying a mixture of a metal alkoxide and a solid lubricant, so the length of time for forming a lubricating film is shortened, the vaporization and discharge of co-hydrolysis products are promoted, and the adhesion of the film becomes strong, so galling resistance is increased.
The present inventors also investigated suitable conditions for forming a phosphate film and a lubricating film of an inorganic polymeric compound having an Mxe2x80x94O backbone on a threaded joint made of carbon steel or a low alloy steel having a Cr content of less than 10 mass percent, and they made the following findings.
(11) Galling can be suppressed by forming a phosphate film on the unthreaded metal contact portion of at least one of either the pin and the box and by further forming a lubricating film of an inorganic polymeric compound having an Mxe2x80x94O backbone atop it.
(12) The mass ratio (B/A) of the content B of the solid lubricant to the content A of the inorganic polymeric compound having an Mxe2x80x94O backbone in the lubricating film is preferably in the range of 0.3-9.0.
(13) Galling resistance is increased by adjusting the surface roughness Rmax of the unthreaded metal contact portion on which the phosphate film is formed to be 3-15 micrometers, and by making the thickness of the lubricating film 5-30 micrometers.
Furthermore, the present inventors investigated suitable conditions for forming a layer of Cu plating and a lubricating film of a resin or an inorganic polymeric compound having an Mxe2x80x94O backbone on a threaded joint, and they made the following findings.
(14) Galling can be suppressed by forming a layer of Cu plating on the unthreaded metal contact portion of at least one of either a pin or a box and by further forming a lubricating film of a resin or an inorganic polymeric compound having an Mxe2x80x94O backbone atop it or on the unthreaded metal contact portion of the other member.
(15) The mass ratio (B/A) of the content B of the solid lubricant to the content A of the resin or the inorganic polymeric compound having an Mxe2x80x94O backbone in the lubricating film is preferably in the range of 0.3-9.0.
(16) Galling resistance is improved by adjusting the surface roughness Rmax of the layer of Cu plating to be 3-15 micrometers and by making the thickness of the lubricating film 5-30 micrometers.
(17) The above-described surface roughness can be achieved by so-called blasting treatment in which shots or other hard pieces are blown against the region on which the layer of Cu plating is to be formed (the substrate) or against the layer of Cu plating.
The present inventors also conceived of forming a rust-preventing film in addition to a lubricating film, they performed basic investigations concerning rust-preventing films, and they made the following findings.
(18) A film containing a salt of a carboxylic acid and an alkali metal or an alkali earth metal is in the form of a semi-solid or a solid at room temperature and is effective for rust prevention over long periods. Such a salt will be referred to as a metal salt of a carboxylic acid. Furthermore, when an alkali metal is referred to, it will include an alkali earth metal.
(19) The above-described film has a high saponification number, and the higher its total base number, the more effective it is for long term rust prevention.
(20) A film which includes the above-described salt and further includes a lubricant such as graphite, molybdenum disulfide, or an organic Mo compound decreases the coefficient of friction of the contact surfaces at the time of tightening and can stabilize the tightening torque of a joint at a low value.
Based on the results of the above-described basic investigations, further detailed investigations were carried out with respect to the rust-preventing properties and lubricity of a threaded joint having a lubricating film and a rust preventing film formed thereon, with rust prevention being investigated by a salt spray test specified by JIS-Z2371 and with lubricity being investigated by a ring/block contact-type friction test, and the following knowledge was obtained.
FIG. 2 shows graphs indicating the relationship between the area in which rusting occurred in a salt spray test and the saponification number, the total base number, the content of alkali metal, and the thickness of a rust preventing film. In the figure, (a) is a graph of the area of rust formation and saponification number, (b) is a graph of the area of rust formation and the total base number, (c) is a graph of the area of rust formation and the alkali metal content, and (d) is a graph of the area of rust formation and the thickness of a rust preventing film.
FIG. 3 shows graphs of the relationship between the coefficient of friction in a friction test and the saponification number, the total base number, the alkali metal content, and the thickness of a rust preventing film. In the figure, (a) is a graph showing the coefficient of friction and the saponification number, (b) is a graph showing the coefficient of friction and the total base number, (c) is a graph showing the coefficient of friction and the alkali metal content, and (d) is a graph showing the coefficient of friction and the thickness of a rust preventing film. In FIGS. 2 and 3, the rust preventing film is a solid film containing a metal salt of a carboxylic acid. The standard film forming conditions were an alkali metal content of 10 mass percent, a saponification number of 50 mgKO H/g, an total base number of 50 mgKO H/g, and a film thickness of 10 micrometers.
(21) As shown in FIGS. 2(a)-(c) and FIGS. 3(a)-(c), when the saponification number is 20-150 mgKO H/g, the total base number is 15-100 mgKO H/g, and the alkali metal content is 2-20 mass percent, then the rust preventing film has excellent rust preventing properties and it has good frictional properties with a reduced coefficient of friction.
(22) When forming a rust preventing film containing a metal salt of a carboxylic acid, if the metal salt is diluted with an organic solvent and a rust preventing oil with a viscosity at 40xc2x0 C. of 2-30 cSt is used after dilution, the adhesion of the rust preventing oil is increased.
(23) When forming a rust preventing film atop a lubricating film, as shown in FIG. 2(d) and FIG. 3(d), rust preventing properties and lubricity can both be maintained at good levels by making the thickness of the rust-preventing film 1 micrometer-50 micrometers.
(24) When a lubricating film is formed on one of a box and a pin and a rust preventing film is formed on the other, the above-described properties can be maintained at good levels by making the thickness of the rust preventing film 5 micrometers-50 micrometers.
(25) If a lubricant in the form of graphite or molybdenum disulfide or an organic Mo compound is included in the rust preventing film, the coefficient of friction is stabilized at an even lower level, and the tightening torque can be stabilized at an even lower level. The content thereof is 5-30 mass percent.
The present inventors performed further investigations based on this knowledge, and they completed the present invention.
Here, the present invention is a threaded joint for an oil well pipe comprising a pin and a box each having a threaded portion and an unthreaded metal contact portion, characterized by having a lubricating film formed on the unthreaded metal contact portion of at least one of either the pin or the box, the lubricating film comprising a solid lubricant dispersed and mixed in an inorganic polymeric compound, the inorganic polymeric compound having an Mxe2x80x94O backbone. This will be referred to as the first invention.
From another standpoint, the present invention is a threaded joint for an oil well pipe comprising a pin and a box each having a threaded portion and an unthreaded metal contact portion, characterized by having a phosphate film formed on the unthreaded metal contact portion of at least one of either the pin or the box, and having a lubricating film formed atop the phosphate film, the lubricating film comprising a solid lubricant dispersed and mixed in an inorganic polymeric compound, the inorganic polymeric compound having an Mxe2x80x94O backbone. This will be referred to as the second invention.
From another standpoint, the present invention is a threaded joint for an oil well pipe comprising a pin and a box each having a threaded portion and an unthreaded metal contact portion, characterized by having a layer of Cu plating formed on the unthreaded metal contact portion of at least one of either the pin or the box, and having a lubricating film formed atop the layer of Cu plating, the lubricating film comprising a solid lubricant dispersed and mixed in a polymeric compound, the polymeric compound comprising an inorganic polymeric compound, the inorganic polymeric compound having an Mxe2x80x94O backbone. The polymeric compound can be a resin. This will be referred to as the third invention.
From another standpoint, the present invention is a threaded joint for an oil well pipe comprising a pin and a box each having a threaded portion and an unthreaded metal contact portion, characterized by having a layer of Cu plating formed on the unthreaded metal contact portion of one of either the pin or the box, and having a lubricating film formed on the other unthreaded metal contact portion, the lubricating film comprising a solid lubricant dispersed and mixed in a polymeric compound, the polymeric compound comprising an inorganic polymeric compound, the inorganic polymeric compound having an Mxe2x80x94O backbone. The polymeric compound can be a resin. This will be referred to as the fourth invention.
From another standpoint, the present invention is a threaded joint for an oil well pipe comprising a pin and a box each having a threaded portion and an unthreaded metal contact portion, characterized by having a lubricating film formed on one of either the pin or the box, and having a rust preventing film formed on the other or atop the lubricating film, the rust preventing film comprising a rust preventing oil composition containing an alkali metal salt of a carboxylic acid and/or an alkali earth metal salt of a carboxylic acid, the lubricating film comprising a solid lubricant dispersed and mixed in a polymeric compound, the polymeric compound comprising an inorganic polymeric compound, the inorganic polymeric compound having an Mxe2x80x94O backbone. The polymeric compound can be a resin. This will be referred to as the fifth invention.
In the threaded joint for an oil well pipe according to the fifth invention, the thickness of the rust preventing film formed on the other is preferably 5-50 xcexcm.
From another standpoint, the present invention is a threaded joint for an oil well pipe comprising a pin and a box each having a threaded portion and an unthreaded metal contact portion, characterized by having a lubricating film formed on both the box and the pin, and having a rust preventing film atop the lubricating film of at least one of either the box or the pin, the rust preventing film comprising a rust preventing oil composition containing an alkali metal salt of a carboxylic acid and/or an alkali earth metal salt of a carboxylic acid, the lubricating film comprising a solid lubricant dispersed and mixed in a polymeric compound, the polymeric compound comprising an inorganic polymeric compound, the inorganic polymeric compound having an Mxe2x80x94O backbone. The polymeric compound can be a resin. This will be referred to as the sixth invention.
In the threaded joint for an oil well pipe according to the sixth invention, the thickness of the rust preventing film formed atop the lubricating film is preferably 1-50 xcexcm.
In the threaded joint for an oil well pipe according to the fifth and sixth inventions, the saponification number of the rust preventing oil composition is preferably 20-150 mgKO H/g, its total base number is preferably 15-100 mgKO H/g, and the total content of alkali metals and alkali earth metals in the rust preventing oil composition is preferably 2-20 mass %.
In the threaded joint for an oil well pipe according to the first through sixth inventions, the inorganic polymeric compound preferably has a Ti-O backbone.